FLUVIAL GEOCHEMISTRY OF THE EASTERN SLOPE OF THE NORTHEASTERN ANDES AND ITS FOREDEEP IN THE DRAINAGE OF THE ORINOCO IN COLOMBIA AND VENEZUELA

The fluvial geochemistry of the tributaries of the Orinoco draining th e eastern branch of the northern Andes in Colombia and Venezuela is de termined by lithology and ranges from rivers dominated by aluminosilic ate weathering, mainly of shales and mafic rocks, to those bearing the signatures of dissolution of marine limestones and evaporites and of continental playa deposits. These left bank tributaries of the Orinoco provide the overwhelming proportion of the suspended and dissolved lo ads transported by the main stem although feeding only half the discha rge. However, due to the extreme severity of weathering on the Guayana Shield, the right bank tributaries supply about half the silica and 4 0% of the K carried by the Orinoco. There is a similar partitioning be tween the Andes and the lowlands in the Amazon drainage. In contrast t he Mackenzie, which drains the northern extremity of the continental a re of the Western Americas, is completely dominated by the weathering of limestones and evaporites and transports very Little silica. This, again, appears to be a reflection of lithology since the Yukon? in sim ilar latitudes and terrain, has silica concentrations comparable to th e Tropical systems. In addition, a conservative deconvolution of the d ata gives estimates of the net consumption of atmospheric CO2 by alumi nosilicate weathering that follows the same pattern. Thus, the availab le evidence from the Western Americas indicates that exposure and lith ology, rather than the climatic variables, temperature, and runoff, do minate the weathering yields in active orogenic belts. These observati ons contradict the conventional, view of weathering processes and thei r controls and are at complete variance with the assumptions parameter ised in most models of the geochemical cycle.